Medical engineering & physics
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The critical closing pressure (CrCP) of the cerebral circulation indicates the value of arterial blood pressure (ABP) at which cerebral blood flow (CBF) approaches zero. Measurements in animals and in humans, have shown that the CrCP is significantly greater than zero. A simple mathematical model, incorporating the effects of arterial elasticity and active wall tension, shows that CrCP can be influenced by several structural and physiological parameters, notably intracranial pressure (ICP) and active wall tension. ⋯ Estimates of apparent CrCP have been shown to be influenced by arterial PCO2, ICP, cerebral autoregulation, intra-thoracic pressure, and mean ABP. There is a lack of investigation, under well-controlled conditions, to assess whether CrCP is altered in disease states. Studies of the cerebral circulation need to take CrCP into account, to obtain more accurate estimates of cerebrovascular resistance changes, and to reflect the correct dynamic relationship between instantaneous ABP and CBF.
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Comparative Study Clinical Trial
Dynamic cerebral autoregulation assessment using an ARX model: comparative study using step response and phase shift analysis.
Middle cerebral arterial blood velocity (MCAv) response to spontaneous and manipulated changes of arterial blood pressure (ABP) was studied in eight subjects using a linear autoregressive with exogenous input (ARX) model. ABP and MCAv were measured non-invasively by photoplethysmograph and transcranial Doppler ultrasound, respectively. Data were recorded at rest (spontaneous changes in ABP) and during thigh cuff (step-wise changes) and lower body negative pressure (sinusoidal changes of 1/12 Hz) tests in both normocapnia and hypercapnia (5% CO2). ⋯ ABP and MCAv were fitted by ARX models and dynamic cerebral autoregulation was estimated by analysing both the step responses and phase shift at the 1/12 Hz of the corresponding ARX models. The ARX model consistently modelled the phase lead of MCAv to ABP and it showed that the phase shift at 1/12 Hz of ARX model is consistent with the real phase shift of the data (p=0.59). Strong linear relationships between pCO2 and gradient of the step response (r=-0.58, p<0.0001) and between pCO2 and phase shift (r=-0.76, p<0.0001) were observed, which suggests that cerebral autoregulation can be assessed by step response or phase shift analysis of the ARX model fitted to ABP and MCAv data with spontaneous changes.
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Comparative Study
Quantitative assessment of cerebral autoregulation from transcranial Doppler pulsatility: a computer simulation study.
Transcranial Doppler (TCD) ultrasonography is largely used today to achieve non-invasive assessment of cerebral autoregulation and cerebrovascular reactivity in neurosurgical patients. Recent experimental and clinical studies suggest that not only the pattern of mean velocity, but also velocity pulse amplitude alterations during changes in cerebral perfusion pressure (CPP) contain information on autoregulation status. The aim of this work is to investigate the relationship between cerebral autoregulation and TCD pulsatility by means of a comprehensive mathematical model of intracranial dynamics and cerebrovascular regulation. ⋯ Starting from these results, we suggest a new quantitative index to assess autoregulation strength, i.e. G(aut)% = (s-b)/a, where G(aut)% is autoregulation strength (100% means intact autoregulation, 0% means impaired autoregulation), a approximately -0.03; b approximately 1.5 and s is the slope of the relationship ' percentage changes of velocity pulse amplitude to arterial pressure pulse amplitude vs. CPP changes'.